Researchers have succeeded in making salmon couples give birth to trout — using a technique that they argue could help to preserve rare species of fish.

Goro Yoshizaki and his colleagues at the Tokyo University of Marine Science and Technology in Japan had previously shown that male salmon could be injected with cells from closely-related trout to produce viable trout sperm. When the sperm were introduced to trout eggs, healthy trout offspring were produced (see 'Salmon give birth to trout').

Now the researchers have taken the work a step further, showing that salmon can be not only the biological fathers but also the mothers of trout offspring. The new work, published in Science1, shows how two sterile masu salmon (Oncorhynchus masou) can together produce nothing but healthy rainbow trout (O. mykiss).

The technique relies on the injection of trout spermatagonia — the early, stem-cell stage of sperm — into salmon embryos, so that the growing salmon produce trout sperm and eggs. The technique could be very useful for storing back-up genetic material of different fish species that are today under threat, because spermatagonia can be easily cryopreserved, says David Penman, a fish geneticist at the University of Stirling, UK.

With many plants and animals, seeds, sperm and eggs can be cryopreserved to later resurrect a species that has died out. "The problem with gene-banking when you come to fish, is that it's almost impossible with eggs," says Penman. "The eggs are very big, very yolky," he says, which makes them nearly impossible to freeze.

If Yoshizaki's technique is broadly applicable to other fish, it will mean that their eggs don't need to be preserved — they could be made at a later date, in a surrogate fish.

The sperm that turned

Injecting trout spermatogonia into a salmon embryo.

Science

If trout spermatagonia are injected into normal male salmon embryos, the fish will grow up to produce a mix of different types of sperm — some salmon, and some trout. In the team's previous work, when sperm from male salmon treated with primordial germ cells (an even earlier stage of sperm) were used to fertilize trout eggs, just 0.4% of the resulting offspring were healthy trout. The rest were hybrids that did not survive.

To increase this percentage, the team turned to salmon designed to have three sets of chromosomes instead of the usual two, making them sterile. When they are injected with spermatagonia, the only viable sperm the males later produce comes from these injected cells — making them 100% trout.

The group also met with success doing the same thing with sterile female salmon. When the female fish were injected as embryos with spermatagonia, the eggs they produced were all trout eggs.

The fact that an early form of sperm could be used to prompt the growth of eggs shows just how flexible these fish are to this type of treatment, notes Penman. "It's really amazing to see that they injected spermatagonia and got eggs — the fish are very plastic."

When mated, these salmon parents produced healthy trout offspring, which in turn mated to give a healthy second generation.

Save the fish

Yoshizaki and his colleagues hope the technique could be used to help save endangered fish.

"There are lots of species of fish that are threatened," says Penman. He says he thinks that to use this approach properly will mean thinking about cryopreserving a lot of material, to save all species and ensure that resulting populations are not too inbred. And some species are likely to be easier to work with than others, he adds. "If you don't happen to have a closely related recipient [for a surrogate] then you'd struggle. There are some groups where there are lots of close relatives, but some where there are only a few," he says.

"It's better to look at all the aspects of conservation biology — but this will be a useful additional technique, as a backup," Penman says.